Newly-promoted Second Lieutentant Brian Ash joins his new regiment only to find that they are assigned to bomb disposal in London. Ash is thrown in at the deep end when he and his men are sent to a bomb site.

Ash tackles a bomb that is not considered vital with a new fuse extractor that doesn't work properly. New Lieutenant Machin is placed with Ash to get some on-the-job training. Major Francis criticises Ash after a patriotic magazine article profiles 347 Section.

Ash works on a bomb outside a destroyed office building while the men of 347 Section try to rescue a woman trapped inside and with another bomb right beside her.

Newly-promoted Major Francis takes command of the unit and proceeds to enforce new disciplines, but his chief target seems to be Ash and his men.